100	PRECIOUS STONES
	comparison, it was found that sunshine affected them most; then brilliant acetylene gas, which was more effective still when tinted yellow by being passed through yellow glass. The electric arc was not so effective, but the electric light of the mercury-vapour lamp, though causing little change at the first, after a few hours' exposure rapidly bleached certain of the colours, whilst having no effect on others. Coal gas with incandescent fibre mantle was slightly effective, whilst the coal-gas, burned direct through an ordinary burner, affected very few of the colours, even after twenty-four hours' exposure at a distance of three feet. In all these cases, though the colours were slightly improved by the stones being kept for a time in the dark, they failed to recover their original strength, showing permanent loss of colour. The Silicates. The chief of these are the garnets, crystallising in the cubic system, and anhydrous. The garnet is usually in the form of a rhombic dodecahedron, or as a trisocta-hedron (called also sometimes an icosatetrahedron), or a mixture of the two, though the stones appear in other cubic forms. In hardness they vary from 6-1/2 to 8-1/2. They average from 40 to about 42 per cent, of silica, the other ingredients being in fairly constant and definite proportions. They are vitreous and resinous in their lustre and of great variety of colour, chiefly amongst reds, purples, violets, greens, yellows and blacks, accord­ing to the colouring matter present in their mass. There are many varieties which are named in accordance with one or more of their constituents, the best known being :